The present invention relates to a lithium secondary battery in which lowering of its power is small even in the case where a depth of discharge becomes deep, its internal resistance is low, and its charging and discharging properties are excellent, and particularly which is suitably used as a battery for driving a motor of an electric vehicle or the like.
In recent years, to cope with a raise in an environmental protection campaign, in the automobile industry, instead of a vehicle using fossil fuels, such as a gasoline vehicle, for the purpose of promoting the introduction of an electric vehicle (EV) or a hybrid electric vehicle (HEV), a battery for driving a motor, which holds the key to making the EV fit for practical use, has been diligently developed.
As a battery for the EV and HEV, in recent years, attention has been paid to a lithium secondary battery having a large energy density, which can lengthen the mileage on one charge as compared with a case where a conventional lead-acid storage battery or a nickel-hydrogen battery is used.
In the lithium secondary battery, a lithium compound is used for a positive active material, and various carbon materials are used for a negative active material. At charging, lithium ions in the positive active material move into the negative active material, and at discharging, the lithium ions captured by the negative active material move to the positive active material, so that charging and discharging are carried out.
The structure of an internal electrode body as a place of a battery reaction in such a lithium secondary battery is roughly divided into a winding type and a lamination type. The internal electrode body of the winding type is fabricated in such a manner that, as shown in FIG. 4, a positive electrode 2 and a negative electrode 3 are wound through a separator 4, and a tab 5 as a current collecting lead is attached to each of the positive electrode 2 and the negative electrode 3. The cylindrical internal electrode body 1 as well as an electrolyte is contained and sealed in a cylindrical container so that an electric cell is fabricated. The structure of an electric cell using such a winding type internal electrode body 1 is suitable for fabricating a compact battery while using a large area electrode (positive electrode and/or negative electrode). In this winding type internal electrode body 1, at least one is sufficient for the number of tabs 5 from each of the positive and negative electrode plates 2 and 3, and even in the case where current collecting resistance from the respective positive and negative electrode plates 2 and 3 is desired to be made small, it is sufficient if the number of tabs 5 is increased, so that the winding type has features that the structure of the inside of the battery does not become complicated, and assembly of the battery is easy.
On the other hand, as shown in FIG. 5, a lamination type internal electrode body 7 is formed by alternately laminating a positive electrode 8 and a negative electrode 9 through a separator 10, and even in the case where an area to one of the positive electrodes 8 and the like is not large, the electrode area of the entire of the battery can be made large by laminating a plurality of electrodes. The shape of the fabricated internal electrode body 7 can be freely designed into, for example, a rectangular parallelepiped type, a disk type, or a cylinder type, by means of the shape of the positive and negative electrodes 8 and 9 and the number of laminated electrodes, so that the lamination type is suitable for the use of the case where the shape of a battery is restricted. However, since a tab 6 is necessary for each of the positive and negative electrode 8 and 9, in addition to other reasons, the structure of the inside of the battery becomes complicated, so that the winding type is superior from the viewpoint of assembly working of the battery.
Even if any battery structure is adopted, since the terminal voltage of the lithium secondary battery is about 4 V, an aqueous solution type electrolyte can not be used, and an organic type electrolyte having lithium ion conductivity lower than that of the aqueous solution type electrolyte must be used. Thus, the internal resistance of a battery is apt to become large. However, in a battery for an EV or an HEV, since the internal resistance and power characteristics of the battery mainly determine the acceleration performance, climbing performance, and the like, it becomes important to make the internal resistance of the battery small and to stabilize the power characteristics.
With respect to the lithium secondary battery as a battery for an EV, for example, “Automobile Technology Association, Collection of Preprints for Academic Lecture 971 (1997)”, pp. 53-56 discloses characteristics of a lithium secondary battery in which LiCoO2 is used for a positive active material and hard carbon is used for a negative active material.
In the publication, as the discharge characteristics of the battery, a discharge specific power at a time when 10 seconds has elapsed in each depth of discharge (hereinafter referred to as DOD) is disclosed. The publication discloses that in the case where 4 v is made a full charge, when the DOD is 0%, the specific power is 1540 W/kg, and when the DOD is 80%, the specific power is 500 W/kg, which indicates that the specific power when the DOD is 80% is only about 32% of that when the DOD is 0%. And also, in the case where 4.2 V is made a full charge, when the DOD is 0%, the specific power is 1740 W/kg, and when the DOD is 80%, the specific power is 620 W/kg, which indicates that the specific power when the DOD is 80% is about 36% of that when the DOD is 0%.
Like this, in the conventional lithium secondary battery, there is a problem that when the DOD becomes deep, the power is remarkably decreased as compared with the case where the DOD is shallow. As one of the causes, it is conceivable that the diffusion of lithium ions in the positive active material is limited to the lithium plane direction in the crystal structure of LiCoO2 used as the positive active material, and the lithium ions can diffuse only two-dimensionally, and as a result, the internal resistance becomes large.
That is, it is conceivable that as the DOD becomes deep, sites in LiCoO2 for taking in the lithium ions are decreased, and the diffusion direction of the lithium ions is limited, so that the speed of taking in the lithium ions into LiCoO2 becomes slow, the movement of the lithium ions from the negative electrode to the positive electrode is blocked, the internal resistance value becomes high, and decrease of power is caused. Since the diffusion of the lithium ion naturally occurs from the surface of the LiCoO2 particle, it is conceivable that this phenomenon is remarkable particularly on the surface of the particle. This is also the case with nickel acid lithium (LiNiO2) used for the positive active material similar to LiCoO2.
Like this, in the case where the decrease in power when the DOD is deep is large, by residual capacity of the battery, a difference occurs in acceleration performance where a particularly large power is required. In the case where the acceleration performance is lowered in this way, there is such a fear that a rear-end collision from the back takes place or running of other vehicles is blocked to cause traffic congestion. Thus, in the battery for an EV or HEV, even in the case where the DOD is deep, that is, even in the case where the residual capacity of the battery is small, it is necessary for the battery to exert the designated acceleration performance, and it is necessary to provide the battery in which the increase of internal resistance and the decrease of power, which are caused from change of the depth of the DOD, are small.
On the other hand, although it is also possible to increase the entire capacity of the battery to a degree that an power necessary for obtaining sufficient acceleration performance is obtained even in the case where the DOD is deep, this is disadvantageous in that the space utility of a vehicle becomes deteriorated since the volume of the battery becomes large, the total weight of the vehicle becomes high, coefficient of utilization of the battery becomes inferior, and the cost of the batteries increases.
With respect to LiCoO2 used for the positive active material, the production of Co as a constituent material is not necessarily large in amount even in the world, and is a relatively expensive material, so that the material has a problem in the use as general-purpose parts in view of cost. Moreover, since the countries of origin are limited, the material has also a problem in securing of the raw material, stable supply of products to a market, and the like.